The third generation partnership project (3GPP) is currently working on standardization of the first release of the Third Generation (3G) Long Term Evolution (LTE) concept. In LTE the downlink is based on orthogonal frequency division multiplexing (OFDM) while the uplink is based on a single carrier modulation method known as discrete Fourier transform spread OFDM (DFT-S-OFDM).
A Random Access (RA) procedure is performed by a user equipment (UE) to seek access to and notify its presence in a network. This is done during initial access, when the UE has lost the uplink synchronization or when the UE is in an idle or a low-power mode.
The basic RA Procedure is a four-phase procedure as outlined in FIG. 2:                Phase 1 consists of transmission (step 21) of a random access preamble from the UE 18 to eNode B (eNB) 15, allowing the eNB to estimate the transmission timing of the UE.        Phase 2 consists of the network transmitting (step 22) a timing advance command to correct the uplink timing, based on the timing of arrival measurement in the first step. Uplink resources and a temporary identifier are assigned to the UE.        Phase 3 consists of signaling from the UE to the network using the uplink synchronization channel (UL-SCH) similar to normal scheduled data. A primary function of this message is to uniquely identify the UE. The exact content of this signaling depends on the state of the UE, e.g., whether it is previously known to the network or not. Thus, a Radio Resource Control (RRC) connection request is sent from the UE 18 to eNB 15 (step 23).        Phase 4, the final phase, is responsible for contention resolution in case multiple UEs tried to access the system on the same resource. Thus, a RRC connection setup is done (step 24).        
RA preambles are based on Zadoff-Chu (ZC) root sequences and cyclic shifts of these, giving good time-domain auto-correlation properties allowing for accurate uplink channel estimation.
Four different preamble formats are defined for LTE Frequency Division Duplex (FDD) in 3GPP. For LTE Time Division Duplex (TDD), only one preamble format is defined. Therefore, choosing preamble format for TDD is not described in the present application. The methods described herein are however not limited to only LTE FDD.
The preamble format specifies both the length of the Cyclic Prefix and the number of RA sequence repetitions. The different preamble formats for LTE FDD are shown in the table below and in FIG. 8 (described in more detail further below), in which TCP is the cyclic prefix time and TSEQ is the sequence time, i.e. the time used to transmit the RA sequence(s) in a RA preamble.
Preamble formatTCPTSEQ0 3168 · TS24576 · TS121024 · TS24576 · TS2 6240 · TS2 · 24576 · TS321024 · TS2 · 24576 · TS
The RA preamble format parameter needs to be set in every cell and depends on the cell size and radio conditions. The RA preamble format parameter may be set manually, but in order to find the most suitable parameter setting, time-consuming and costly simulations and drive tests would according to previously known methods have to be performed.
If network characteristics change, e.g. the interference level increases or the antenna tilt is changed, the RA preamble format may need to be changed. With the, in comparison slow, manual processes, the RA preamble format setting will not be sufficiently responsive to changes in network.
There is, therefore, need for improved methods and arrangements, which enable automatic tuning of the RA preamble format.